1. Field of the Invention:
The invention discloses a measured-force gripper device of articulated structure interposed between the contact plate and its support.
2. Description of the Prior Art:
Manipulators are mechanical devices which replace human hands to perform certain operations.
Manipulators are used, for example, to protect an operator against a radioactive atmosphere contained in an enclosure. The manipulations of the operator on a master manipulator disposed outside the enclosure, behind a protected wall, are reproduced by the manipulator disposed inside the enclosure.
Another application of manipulators is to take the place of labor, for example, for repetitive operations.
However, in all applications the precision which can be obtained at the end of the manipulator is limited on the one hand by the imperfections of its mechanical construction, and on the other hand by the capacities of the operator him or herself, when a master manipulator is controlled by an operator.
For example, to introduce a pin into a bore, it must be moved precisely into the axis of the bore. If this condition is not met, there is a risk of spoiling the wall of the bore or the pin which is to be introduced. However, as has been recalled, the precision of a manipulator is not adequate to obtain satisfactory alignment of the member to be inserted with the bore.
To get over this difficulty, measured-force gripper devices have been constructed. Such devices enable the forces exerted on the article which is handled to be measured and compared with a desired value predetermined as a function of the operation to be performed, and the position of the manipulator to be corrected in relation to the difference between the force measured and the required value. For example, in the case already mentioned of the insertion of a pin into a bore, there must be a zero force along two axes perpendicular to the axis of the bore. The force along the axis of the bore is not zero, but it must remain lower than a predetermined value in relation to various parameters, such as the surface state of the bore or the pin, or the clamping of the two members. The manipulator bearing the member to be inserted is therefore repositioned until the force measured in two directions perpendicular to the axis of the bore is zero and the force along such axis less than the required value. From that moment onwards the insertion operation is pursued. If at any moment one of the forces should exceed the required value, insertion would stop. The manipulator would be repositioned, and so on and so forth.
For example, the prior art discloses tongs whose main axis is connected to a cross whose ends are trapped by a circular member connected to the manipulator. The ends of the cross are formed by deformable balls which act as force-detectors. The force exerted on the tongs deforms the balls. From this, information regarding the force can be deduced. Such information is then used to modify the position of the manipulator. U.S. Pat. No. 3,921,445 (HILL et al.) discloses a gripper device of that kind. It comprises tongs whose jaws 29 and 30 are attached to the ends of parallel links 58 to 60. Two of the links are connected to toothed wheels 50 and 52 adapted to be driven in the two directions of rotation by a reversible motor 20.
The assembly formed by the jaws, links and toothed wheels 50, 52 is mounted on a support 22. A ring 18 is attached to the support 22 by screws 24. A number of resilient coupling members 26, attached on the one hand to the ring 18 and on the other to the support 10, attached to the manipulator 8, make a resilient connection between these two members. Means are provided for detecting the deformation of the resilient coupling means. For example, in one embodiment, such means are formed by four elements 25.sub.1 to 25.sub.4, in which light-transmitting holes are made. Certain of these holes comprise on the one hand a light source, and on the other light-detecting means. Spindles 84 connected to the support 10 transmit the relative movement of the member 10 in relation to the bush 18 to the elements 25. The amount of light passing through the passages is a function of the forces and torques exerted.
However, a measured-force gripper device of that kind has a number of disadvantages.
The centre of seizure--i.e., the point lying between the two jaws which is equidistant from the geometrical centre thereof, is a long way from the resilient coupling means. As a result, the coupling means are sensitive to the sum of the weights of the tongs and of the article handled, whereas it would be desirable for them to be sensitive only to the forces exerted on the article.
A second disadvantage is that the resilient coupling means deliver coupled items of information, so that the forces and torques exerted on the article along three orthogonal axes cannot be obtained directly without complex calculations. The tongs can be oriented in a large variety of positions, in dependence on the degrees of freedom which they comprise. The interpretation of the information delivered by the force-detecting device must at every moment take the orientation of the tongs into account, the result being a highly complex calculation.
A third disadvantage is that the coupling means form a rigid structure which does not allow the article handled to move, thus denying it some suppleness and some adaptability, so that the device allows only restricted applications.
Lastly, the information delivered by the coupling means is influenced by the clamping force of the tongs, since when the article is clamped, the torque exerted by the rod 32 and the pinion 34 is transmitted to the support 22. This torque is equilibrated by the resilient coupling means. It is therefore impossible to deduce directly an outside force exerted on the article without taking this clamping force into account, which is itself difficult to determine.
The invention relates to a measured-force gripper device which obviates these disadvantages and more precisely enables information to be delivered which allows the direct determination, without any complex treatment, of the intensity of the force exerted on the article in accordance with each degree of freedom. An article can be displaced in accordance with six degrees of freedom-three degrees of rotary freedom, and three degress of prismatic freedom. A degree of rotary freedom takes the form of a pivoting, while a degree of prismatic freedom takes the form of a translational movement. Hereinafter the expression "degree of freedom" will be taken to mean a translational or rotary movement.